Initial temporal and spatial changes of the refractive index induced by focused femtosecond pulsed laser irradiation inside a glass

The temporal and spatial developments of the refractive-index change in a focal region of a femtosecond-laser pulse inside a soda-lime glass is investigated by the transient lens (TrL) method with a time resolution of subpicosecond. In the TrL signal, the oscillation with about an 800-ps period is observed until about 2000 ps. In order to explain the oscillation, the thermoelastic response of a heated material by a short pulsed laser is calculated. It is found that the TrL signal calculated based on the thermoelastic calculation reproduces the observed oscillating signal very well, even though the calculated density at the focal region does not oscillate. The essential feature of the oscillation can be explained in terms of the pressure wave generation and propagation in the outward direction from the irradiated region. Based on the pressure-wave propagation and the phase-retrieval method, the temporal evolution of the refractive-index distribution inside a glass is obtained from the probe-beam deformation (TrL image) at various delay times between the pump and probe pulses. Two phases of the refractive-index increase at the laser focal region were observed in a range of 20-100 and 500-700 ps, which may cause a permanent refractive-index increase in the laser focal region inside a glass. We discuss the effect of the laser pulse duration on the material deformation process in the laser-irradiated region. This study clearly shows the initial process of the material deformation dynamics inside a glass after femtosecond laser irradiation.